The present invention relates to a photosensor and more particularly to a photosensor having a fast time response, good sensitivity and high resolution.
A great number of photosensors employing various kinds of photo detecting materials (photosensor elements) have been developed. A line image sensor is one type of photosensor for detecting a one dimensional image and is typically used in facsimile machines. A read-out device for reading out the image in a facsimile machine comprises a line image sensor made of MOS or CCD and a lens combination for projecting the image onto the line image sensor. This kind of read-out device makes the size of the facsimile machine large because of the required lens combination.
Therefore, in order to miniaturize the facsimile apparatus there has been proposed a contact-type line image sensor comprising a CdS-CdSe photo conductive element array or a thin film photosensor element array of, for example, amorphous silicon (Si) photo diodes. The CdS-CdSe photo conductive element array features provision of a large photo current and homogeneous element sensitivity. The use of this array with a matrix driving system simplifies the read-out circuit. Since the CdS-CdSe array has a relatively low photo responsibility, the use of the CdS-CdSe array makes it difficult to get a high transmission velocity with a read-out device in a facsimile machine. The amorphous Si photo diode array has a photo responsibility which is one hundred times as large as that of the CdS-CdSe array. The amorphous photo Si diode array, however, necessarily requires a read-out circuit with a direct driving system having a number of analog switches, resulting in a complex read-out circuit and making it difficult to integrate the respective photo detecting elements in high density and to improve the resolution characteristic.